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May 7, 2007 -- There appear to be almost twice as many Americans with bipolar disorder as previously thought, and many are not getting the treatments they need, researchers from the National Institute of Mental Health report.

Once thought of as a single mental illness, bipolar disorder is increasingly recognized as a spectrum disorder, with symptoms ranging from less severe to devastating.

The NIMH researchers found that people with the mildest form of the condition, often referred to as sub-threshold bipolar disorder, generally sought treatment for other mental health conditions such as depression or substance abuse.

NIMH senior investigator Kathleen R. Merikangas, PhD, says a large percentage of people diagnosed with major depression may actually have this form of bipolar disorder.

"Misdiagnosis is particularly troubling because the drugs used to treat depression can actually trigger bipolar symptoms," she tells WebMD.

What Is Bipolar Disorder?

There are two main types of bipolar disorder (once known as manic depression): bipolar disorder I and bipolar disorder II. Symptoms include dramatic moods swings between euphoria and severe depression; patients may have hallucinations or delusions.

Patients with bipolar I have the most severe symptoms; bipolar II patients have more moderate symptoms.

In 2006, the NIMH estimated that 2.6% of the U.S. population, or roughly 5.7 million American adults, suffered from bipolar disorder in any given year.

By including patients who met the diagnostic criteria for sub-threshold bipolar disorder in their latest analysis, Merikangas and NIMH colleagues concluded that about 4.4% of U.S. adults have some degree of bipolar illness during some point in their lives.

The researchers evaluated data from a nationwide mental disorders survey conducted between February 2001 and April 2003, involving 9,282 adults living in the U.S.

The lifetime incidence of bipolar I and bipolar II was roughly 1% each in the surveyed population and 2.4% for sub-threshold bipolar disorder.

"The [findings] reinforce the argument of other researchers that clinically significant sub-threshold bipolar disorder is at least as common as threshold bipolar disorder," Merikangas and colleagues wrote in the May issue of Archives of General Psychiatry.

Targeting Treatment

Most people who met the clinical definition of sub-threshold bipolar disorder (70%) were already receiving treatment when surveyed. Many were taking antidepressants, according to Merikangas.

Depression, substance abuse, and anxiety disorder are all conditions commonly seen in bipolar disorder patients, complicating the diagnosis of less severe bipolar illness.

As a result, mood-stabilizing drugs such as lithium, which are most effective for treating bipolar illness, are widely underprescribed while antidepressants are being prescribed far too often, Merikangas says.

The researchers conclude that clinicians who treat patients for depression, anxiety, or substance abuse must develop a higher suspicion of bipolar disorder.

"Bipolar disorder can manifest itself in several different ways. But regardless of type, the illness takes a huge toll," NIMH Director Thomas R. Insel, MD, says in a news release.

"The survey's findings reiterate the need for a more refined understanding of bipolar symptoms so we can better target treatment."